1. Field of the Invention
This invention relates to electronic circuits used as voltage regulators and more specifically to circuits and methods used to stabilize a voltage regulator.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in voltage regulation circuits. Voltage regulators are inherently medium to high gain circuits, typically 50 db or greater, with low bandwidth. With this high gain and low bandwidth, stability is often achieved by setting a dominate pole with the load capacitor. Achieving stability over a wide range of load currents with a low value load capacitor (.about.0.1 uF) is difficult because the load pole formed by the load capacitor and load resistor can vary by more than three decades of frequency and be as high as tens of KHz requiring the circuit to have a very broad band of greater than 3 MHz which is incompatible with the power process used for voltage regulators.
FIG. 1 shows a prior art solution to the stabilization problem. The voltage regulator 24 in FIG. 1 converts an unregulated Vdd voltage, 12 volts in this example, into a regulated voltage at node 26, 5 volts in this example. Capacitor 8, resistor 10, amplifier 12, and resistor 14 are configured as an integrator having the output voltage node 26 as an inverting input and a voltage reference as the non-inverting input. The integrator drives bipolar transistor 4 which is connected in series with an output current mirror formed by p-channel transistors 2 and 16, as is known in the art. Resistor 18 is a pull down resistor added to increase the stability of the circuit.
In this prior art example, the pole associated with the pull down resistor can be calculated as: EQU f=1/2.pi.R.sub.L C.sub.L
where
R.sub.L =resistance of the load=R18 in parallel with R20 and PA1 C.sub.L =is typically around 0.1 microfarad
Therefore, the pole associated with the prior art circuit is load dependent and can vary from 16 Hz to 32 KHz for an R18 equal to 100 kilo-ohms and R20 ranging from 50 ohms to 1 mega-ohm. The wide variation of the pole frequency is difficult to stabilize, as will be appreciated by persons skilled in the art. A prior art solution to this problem is to change the pull down resistor R18 from 500 kilo-ohms to around 500 ohms which changes the pole frequency to a range of 3.2 KHz to 32 KHz, which is a frequency spread of 1 decade instead of 3 decades. However, the power dissipated by the output transistor 16 increases, as shown below: EQU power=(12 v-5 v)(I.sub.load +I.sub.pull down)=(7 v)(100 mA)+(7 v)(10 mA)
Therefore, the 500 ohm resistor adds 70 milli-watts of power dissipation in the chip which is approximately a 10% increase in power dissipation for the added stability.